Polyurethane elastic fiber is stretchable fiber excellent in elastic function and finds use in a wide variety of fields including clothing materials such as foundation garments, socks, pantyhose, swimming wear, sportswear and leotards; and non-clothing materials such as diapers, bandages, supporters, covering masks, automobile interior materials, nets and tapes.
Polyurethane elastic fiber is elastic fiber primarily constituted of segment polyurethane and basically constituted of block copolymers mainly containing high molecular-weight polyol, diisocyanate and a chain extender. In view of chemical structure, polyurethane elastic fiber is constituted of a soft segment excellent in flexibility and a hard segment having a crystalline structure formed by strong intermolecular force ascribed from hydrogen bonding. Furthermore, depending upon the type of chain extender constituting the hard segment, polyurethane elastic fibers can be classified into polyurethane-urea type, which uses a low-molecular diamine and has a urea bond, and polyurethane-urethane type, which uses a low molecular-weight polyol and has a urethane bond. The hydrogen bonding force of the hard segment has a large effect upon physical properties such as heat resistance. Since the hydrogen bonding force of the urea bond is stronger than that of the urethane bond, the polyurethane-urea type is superior in heat resistance and is in a main stream of polyurethane elastic fibers currently produced. For these reasons, the polyurethane-urea type is used in a wide variety of fields. In the present invention, the elastic fiber containing such a polyurethane-urea type polymer (hereinafter referred to as “polyurethane urea polymer”) as a main component will be referred to as a “polyurethane urea elastic fiber”. On the other hand, elastic fiber formed of a polyurethane-urethane type polymer is inferior in heat resistance and recoverability compared to polyurethane urea elastic fiber but has setting property at relatively low temperature. Taking advantage of this property, conversely, the elastic fiber formed of a polyurethane-urethane type polymer is applied to, for example, wool woven fabrics and zokki-type pantyhose.
When polyurethane urea elastic fiber is used in the general field of clothing materials, it is usually interknitted with polyamide fiber, polyester fiber, cotton etc., and subjected to manufacturing steps including cutting-out, sewing and finish processes to obtain clothing items. When the fabric interknitted using polyurethane urea elastic fiber is cut out and sewed, the fabric sometimes curls and frays at the edge depending upon the design of the fabric due to its high heat resistance and recoverability, rendering sewing difficult. Furthermore, polyurethane urea elastic fiber is removed from the knitted structure of cloth at the frayed hem, reducing stretchability of the portion of the cloth.
Since the edge of cloth will be frayed if it is allowed to stand as it is after cutting, the edge of general clothing items is more or less treated by any means to prevent fray. For example, generally, the cut edge is fold, doubled and sewed, or wrapped with another cut-cloth such as a tape and sewed. However, post-processing work for preventing fray such as hemming and sewing takes time and labor in a process for producing clothing items and also economically provides a big burden. In addition, a clothing item hemmed and sewed at the edge becomes thick to form a stepped portion at the hem. In the case of underwear items such as foundation, the stepped portion protrudes when a person puts outerwear on the underwear, and appears as a projection of the outerwear, impairing appearance. Furthermore, polyurethane urea elastic fiber is frequently used in clothing items such as foundation garments and pantyhose which directly and tightly fit to a body. The thick hem makes the person uncomfortable to wear.
To overcome problems such as hemmed and sewed edges of clothing using polyurethane urea elastic fiber in the field of foundation garments such as brassieres, girdles and body suits, in which fashionable design items have been increasingly produced nowadays, a method for manufacturing a clothing item having a so-called cut-off opening parts has been studied to prevent a underwear line from appearing on outerwear by leaving the cut edge alone without hemming and sewing.
For example, a clothing item using fabric requiring no hemming has been proposed (see, for example, PATENT DOCUMENT 1). The clothing item is formed of knitted warp fabric, which is a 1×1 knit construction having the non-elastic fiber and elastic fiber run side by side and at least one side of the non-elastic fiber and the elastic fiber forms closed stitches at each knitting needle.
However, in this case of PATENT DOCUMENT 1, since fray of the cut edge is structurally suppressed by fabric design, the entire cloth becomes thick. Likewise, cloth obtained by fabric design is limited and uses of clothing items thereof are limited.
Furthermore, a clothing item also having a cut-off opening parts has been proposed (see PATENT DOCUMENT 2 or 3), which is obtained by using a low-melting point polyurethane elastic fiber composed of a polyurethane-urethane type in combination with another type of fiber and knitting them in the form of plating stitch and applying heat-set processing to impart a function of preventing fray.
However, in the polyurethane elastic fiber of a polyurethane-urethane type, physical properties significantly decrease by heat, which is applied in a setting step for fixing the shapes of fabric and clothing items and in a dyeing step. In addition, recoverability of fabric decreases and also breakage of polyurethane elastic fiber may occur in the temperature conditions usually employed for processing polyurethane urea elastic fiber. Consequently, clothing items employing this fabric is thermally limited in processing conditions.
Furthermore, a method for producing an stretchable fiber structural product rarely fraying is proposed (see PATENT DOCUMENT 4), which is obtained by subjecting a fiber structure, which uses polyurethane elastic fiber spun from a spinning solution containing at least two types of polyurethane components different in melting point at the high temperature side (for example, a polyurethane-urethane type and a polyurethane-urea type) to heat treatment performed at not less than a heat deformation temperature of the polyurethane component having a lower high-temperature side melting point.
Nevertheless, it cannot be said that the effect of suppressing fray of the fabric obtained in this manufacturing method is sufficiently satisfactory compared to the aforementioned case of using low-melting point polyurethane elastic fiber. In addition, no consideration is given to possibility of decreasing basic performances of elastic fiber, such as high recoverability and extensibility that a polyurethane-urea type possesses by adding not less than two types of polyurethane components different in structure.    PATENT DOCUMENT 1: JP-A-2003-147618    PATENT DOCUMENT 2: JP-A-2005-113349    PATENT DOCUMENT 3: JP-A-2005-350800    PATENT DOCUMENT 4: JP-A-2005-330617